1. Field of the Invention
This invention relates to a method of coloring tubular food casings and more particularly to a method of imparting a natural color to cellulose and collagen casings.
2. The Prior Art
Tubular food casings for edible products such as weiners, sausages, bolognas, and the like, are presently manufactured, on a commercial basis, from regenerated cellulose and collagen. These casings generally are made from aqueous solutions of the base materials, extruded in a coagulation bath, formed into a tube, then inflated with air and passed through a dryer. These tubular films, in the gel state, before drying, generally contain a substantial proportion of water, e.g., about 65% by weight for a regenerated cellulose casing and about 91% by weight for tanned collagen casings.
Cellulose casings are normally prepared by extruding a viscose solution through an annular die into a coagulating bath to produce a hollow, thin-walled tube of coagulated and partially regenerated cellulose. The tube is subsequently treated in an acid bath to thoroughly regenerate the cellulose and washed to remove by-products. The regenerated cellulose tube in the gel state is treated with an aqueous solution of a plasticizer such as glycerine to effect softening thereof and then dried in a dryer at a temperature of about 80.degree.-125.degree. C. while inflated under an air pressure of about 6 psi for 3 to 10 minutes. After drying, the casing is wound on reels and subsequently shirred on high speed shirring machines.
Cellulose casings having a fibrous reinforcement are prepared wherein a tube of fibrous paper, preferably a long fiber hemp paper, is formed and passed adjacent to the annular orifice of the extrusion die so that the paper tube is impregnated and coated with viscose. The impregnated and coated tube is passed through a coagulating and regenerating bath to produce regenerated cellulose within the fibrous paper and on the surface of the paper as a substantial coating. The final product, known in the trade as fibrous casing, consists of about 35 -40% fibrous paper and about 60-65% regenerated cellulose and softening components. The fibrous paper after regeneration of the viscose is washed to remove impurities and then dried in a manner similar to that described for the unreinforced or clear casing.
In the manufacture of collagen casings, a collagen source, typically animal hide collagen, is converted into a finely divided fibrillar form, swelled in water at a pH between 2.5 and 3.7 and extruded in the form of a dilute (2-6% by weight) collagen slurry through a die to form a tubular casing. The extruded collagen is passed into a coagulation bath which dehydrates the collagen slurry into a gel-like collagen casing. The coagulating bath typically contains a salt such as sodium sulfate or ammonium sulfate in large concentration (e.g. 40% by weight) and a small amount of alkali, e.g., sodium hydroxide or ammonia, for neutralizing excess acid which may be present in the collagen casing. The collagen casing, then, is hardened or tanned to form a more coherent structure and permit further processing of the casing, i.e., to provide sufficient strength so that it can be threaded and inflated during drying. Often, a two-step tanning process is employed, with the first tanning step being effected by an aluminum tanning agent, e.g., an aluminum citrate complex, and the second tanning step being effected by reaction with a dialdehyde, e.g., glutraldehyde. After removal from the tanning bath, the casing is passed through a bath containing a plasticizing agent. The casing removed from the plasticizing bath is in a gel state and contains a substantial amount of water, e.g., 90 to 91% by weight. In order that the casing be used as a sausage casing the plasticized casing is inflated with air at about 0.05 psi and dried in a drier at a temperature of about 65.degree.-90.degree. C. for 2 to 10 minutes to reduce the moisture content to about 14% by weight.
Synthetic food casings derived from cellulose and collagen are frequently colored for esthetic effects. The color frequently desired is a transparent light brown color to imitate the color frequently observed on "natural" casings prepared from the intestines of butchered animals.
In cellulosic casings the "natural" color is frequently imparted to the casing by injecting a concentrated blood solution (of slaughtered animals) into viscose before extrusion. The blood coloring components become entrapped in the casing substrate during coagulation of the viscose which results in a permanent coloring of the casing.
Coloring cellulosic casings with blood presents several problems including special handling of the colorant due to its animal derivation, variation in the colorant from blood components derived from different animal sources, and plugging of extrusion dies due to insoluble materials which may be present in the blood.
A natural color is imparted to collagen casings using various combinations of water soluble food colors such as Orange B and Blue I which are respectively sulfonated arylazo and sulfonated triarylmethane type dyes. A problem encountered with these colorants is the extractibility of these colors when the casing is rewetted or otherwise brought into contact with water which causes unacceptable discoloration of the encased meat product.